JP7769485B2 - Pressure-sensitive adhesive composition, pressure-sensitive adhesive sheet, optical laminate, image display device, and method for producing pressure-sensitive adhesive sheet - Google Patents

Description

The present invention relates to a pressure-sensitive adhesive composition, a pressure-sensitive adhesive sheet, an optical laminate, an image display device, and a method for producing a pressure-sensitive adhesive sheet.

In recent years, image display devices, such as liquid crystal display devices and electroluminescence (EL) display devices (e.g., organic EL display devices and inorganic EL display devices), have rapidly become popular. These various image display devices typically have a layered structure consisting of an image-forming layer, such as a liquid crystal layer or an EL light-emitting layer, and an optical laminate containing an optical film and an adhesive sheet. The adhesive sheet is primarily used to bond between films contained in the optical laminate, or to bond the image-forming layer to the optical laminate. Examples of optical films include polarizing plates, retardation films, and polarizing plates with retardation films, which are a combination of a polarizing plate and a retardation film.

Japanese Patent Application Laid-Open No. 2008-096734

Excessive changes in the dimensions of optical films due to temperature changes can cause light leakage and color unevenness in image display devices. Light leakage and color unevenness are particularly likely to occur in relatively large image display devices that use polarizing plates with retardation films. In addition, as image display devices designed with narrow bezels (narrow bezels) are becoming more common, suppressing dimensional changes is becoming increasingly important.

Increasing the modulus of elasticity of the adhesive sheet included in the optical laminate is one way to suppress dimensional changes. One method for increasing the modulus of elasticity of the adhesive sheet is to increase the amount of crosslinking agent blended in the adhesive composition used to produce the adhesive sheet. For example, Patent Document 1 discloses an adhesive composition that uses a sufficient amount of an isocyanate-based crosslinking agent.

However, the inventors' research has revealed that when a pressure-sensitive adhesive sheet is produced using a pressure-sensitive adhesive composition containing a sufficient amount of isocyanate-based crosslinking agent, the pressure-sensitive adhesive sheet is prone to developing minute depressions. These depressions can become optical defects. Furthermore, simply increasing the elastic modulus of the pressure-sensitive adhesive sheet can reduce the durability of the pressure-sensitive adhesive sheet, making it unable to adapt to dimensional changes.

The present invention therefore aims to provide a pressure-sensitive adhesive composition that contains a sufficient amount of isocyanate-based crosslinking agent and is suitable for producing pressure-sensitive adhesive sheets that are less likely to develop dents and have improved durability.

After extensive research, the inventors discovered that when a pressure-sensitive adhesive sheet is produced using a pressure-sensitive adhesive composition containing a sufficient amount of isocyanate-based crosslinking agent, the gel fraction of the pressure-sensitive adhesive sheet immediately after production (initial gel fraction) is low. Based on this finding, the inventors conducted further research and discovered that the initial gel fraction affects the likelihood of dents forming, and that the initial gel fraction can be increased by appropriately combining crosslinking agents, thereby completing the present invention.

The present invention provides
a (meth)acrylic polymer (A);
an isocyanate-based crosslinking agent in an amount of 2 parts by weight or more based on 100 parts by weight of the (meth)acrylic polymer (A);
a peroxide-based crosslinking agent;
The present invention provides a pressure-sensitive adhesive composition comprising:

Furthermore, the present invention provides
A pressure-sensitive adhesive sheet formed from the pressure-sensitive adhesive composition is provided.

Furthermore, the present invention provides
An optical laminate is provided, which includes the pressure-sensitive adhesive sheet and an optical film.

Furthermore, the present invention provides
An image display device including the above optical laminate is provided.

Furthermore, the present invention provides
applying the pressure-sensitive adhesive composition to a substrate to form a coating film;
drying the coating film;
The present invention provides a method for producing a pressure-sensitive adhesive sheet, comprising:

The present invention provides a pressure-sensitive adhesive composition that contains a sufficient amount of an isocyanate-based crosslinking agent and is suitable for producing a pressure-sensitive adhesive sheet that is less likely to develop dents and has improved durability.

FIG. 1 is a cross-sectional view schematically showing an example of the pressure-sensitive adhesive sheet of the present invention. FIG. 2 is a cross-sectional view schematically showing an example of the optical layered body of the present invention. FIG. 3 is a cross-sectional view schematically showing an example of the optical layered body of the present invention. FIG. 4 is a cross-sectional view schematically showing an example of the optical layered body of the present invention. FIG. 5 is a cross-sectional view schematically showing an example of the optical layered body of the present invention. FIG. 6 is a cross-sectional view schematically showing an example of an image display device of the present invention.

The present invention is described in detail below, but the present invention is not limited to the following embodiments and can be modified and implemented as desired without departing from the spirit of the present invention.

(Embodiments of Pressure-Sensitive Adhesive Composition)
The pressure-sensitive adhesive composition of the present embodiment includes a (meth)acrylic polymer (A), an isocyanate-based crosslinking agent and a peroxide-based crosslinking agent in an amount of 2 parts by weight or more per 100 parts by weight of the (meth)acrylic polymer (A). A pressure-sensitive adhesive sheet can be produced from the pressure-sensitive adhesive composition.

[(Meth)acrylic polymer (A)]
The (meth)acrylic polymer (A) can function as a base polymer for an acrylic pressure-sensitive adhesive. Acrylic pressure-sensitive adhesives tend to have excellent optical transparency, appropriate adhesive properties such as wettability, cohesion, and adhesion, and excellent weather resistance, heat resistance, etc. The (meth)acrylic polymer (A) contains, for example, a structural unit derived from alkyl (meth)acrylate as a main component. In this specification, "(meth)acrylate" means acrylate and/or methacrylate. "Main component" means the structural unit that is contained in the largest amount by weight of all structural units that constitute the polymer.

The number of carbon atoms in the alkyl group contained in the alkyl (meth)acrylate that forms the main skeleton of the (meth)acrylic polymer (A) is not particularly limited and may be, for example, 1 to 30. This alkyl group may be linear, branched, or cyclic. Examples of the alkyl group include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, amyl, hexyl, cyclohexyl, heptyl, 2-ethylhexyl, isooctyl, nonyl, decyl, isodecyl, dodecyl, isomyristyl, lauryl, tridecyl, pentadecyl, hexadecyl, heptadecyl, and octadecyl groups. Alkyl (meth)acrylates may be used alone or in combination. The average number of carbon atoms in the alkyl group is preferably 3 to 9. The alkyl (meth)acrylate is preferably butyl acrylate.

In (meth)acrylic polymer (A), the content of structural units derived from alkyl (meth)acrylate is, from the viewpoint of improving the adhesiveness of the pressure-sensitive adhesive sheet, for example, 50% by weight or more, preferably 60% by weight or more, more preferably 70% by weight or more, and even more preferably 80% by weight or more.

In addition to alkyl (meth)acrylates, the monomers constituting the (meth)acrylic polymer (A) include at least one copolymerizable monomer selected from the group consisting of aromatic ring-containing monomers, amide group-containing monomers, carboxyl group-containing monomers, and hydroxyl group-containing monomers. The copolymerizable monomers can be used alone or in combination.

The (meth)acrylic polymer (A) preferably contains structural units derived from an aromatic ring-containing monomer. The aromatic ring-containing monomer is a compound that contains an aromatic ring structure and a polymerizable unsaturated double bond such as a (meth)acryloyl group or a vinyl group. Examples of aromatic rings include a benzene ring, a naphthalene ring, and a biphenyl ring. The aromatic ring-containing monomer is preferably an aromatic ring-containing (meth)acrylate.

Examples of aromatic ring-containing (meth)acrylates include benzyl (meth)acrylate, phenyl (meth)acrylate, o-phenylphenol (meth)acrylate, phenoxy (meth)acrylate, phenoxyethyl (meth)acrylate, phenoxypropyl (meth)acrylate, phenoxydiethylene glycol (meth)acrylate, ethylene oxide-modified nonylphenol (meth)acrylate, ethylene oxide-modified cresol (meth)acrylate, phenol ethylene oxide-modified (meth)acrylate, 2-hydroxy- Examples include those having a benzene ring, such as 3-phenoxypropyl (meth)acrylate, methoxybenzyl (meth)acrylate, chlorobenzyl (meth)acrylate, cresyl (meth)acrylate, and polystyryl (meth)acrylate; those having a naphthalene ring, such as hydroxyethylated β-naphthol acrylate, 2-naphthoethyl (meth)acrylate, 2-naphthoxyethyl acrylate, and 2-(4-methoxy-1-naphthoxy)ethyl (meth)acrylate; and those having a biphenyl ring, such as biphenyl (meth)acrylate. Of these, benzyl (meth)acrylate and phenoxyethyl (meth)acrylate are preferred, with benzyl acrylate being more preferred, from the viewpoint of improving the adhesive properties and durability of the pressure-sensitive adhesive sheet.

The (meth)acrylic polymer (A) may contain structural units derived from an amide group-containing monomer. The amide group-containing monomer is a compound that contains an amide group in its structure and a polymerizable unsaturated double bond such as a (meth)acryloyl group or a vinyl group. Examples of the amide group-containing monomer include acrylamide-based monomers such as (meth)acrylamide, N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N-isopropylacrylamide, N-methyl(meth)acrylamide, N-butyl(meth)acrylamide, N-hexyl(meth)acrylamide, N-methylol(meth)acrylamide, N-methylol-N-propane(meth)acrylamide, aminomethyl(meth)acrylamide, aminoethyl(meth)acrylamide, mercaptomethyl(meth)acrylamide, and mercaptoethyl(meth)acrylamide; N-acryloyl heterocyclic monomers such as N-(meth)acryloylmorpholine, N-(meth)acryloylpiperidine, and N-(meth)acryloylpyrrolidine; and N-vinyl group-containing lactam-based monomers such as N-vinylpyrrolidone and N-vinyl-ε-caprolactam. Among these, N-vinyl group-containing lactam monomers are preferred from the viewpoint of improving the durability of the pressure-sensitive adhesive sheet.

The (meth)acrylic polymer (A) may contain structural units derived from a carboxyl group-containing monomer. A carboxyl group-containing monomer is a compound that contains a carboxyl group in its structure and a polymerizable unsaturated double bond such as a (meth)acryloyl group or a vinyl group. Examples of carboxyl group-containing monomers include (meth)acrylic acid, carboxyethyl (meth)acrylate, carboxypentyl (meth)acrylate, itaconic acid, maleic acid, fumaric acid, and crotonic acid. Among these, acrylic acid is preferred from the viewpoints of copolymerizability, cost, and improved adhesive properties of the PSA sheet. Having structural units derived from a carboxyl group-containing monomer, particularly acrylic acid, in the (meth)acrylic polymer (A) can enhance, for example, the self-polymerization of an isocyanate-based crosslinking agent. Improving the self-polymerization properties of the isocyanate-based crosslinking agent can particularly contribute to preventing peeling of the PSA sheet in humid environments and stabilizing the physical properties of the PSA sheet in systems with a high content of isocyanate-based crosslinking agent.

The (meth)acrylic polymer (A) may contain structural units derived from a hydroxyl group-containing monomer. The hydroxyl group-containing monomer is a compound that contains a hydroxyl group in its structure and a polymerizable unsaturated double bond such as a (meth)acryloyl group or a vinyl group. Examples of hydroxyl group-containing monomers include hydroxyl group-containing alkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, and 12-hydroxylauryl (meth)acrylate; and hydroxyl group-containing cycloalkyl (meth)acrylates such as (4-hydroxymethylcyclohexyl)-methyl acrylate. Among these, 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate are preferred.

Among copolymerizable monomers, aromatic ring-containing monomers and carboxyl group-containing monomers are preferred from the standpoint of adhesiveness, durability, etc., with aromatic ring-containing monomers being particularly preferred. (Meth)acrylic polymers (A) containing structural units derived from carboxyl group-containing monomers tend to promote the reaction between isocyanate-based crosslinking agents by incorporating water molecules from the surrounding atmosphere. Aromatic ring-containing monomers improve the compatibility between (meth)acrylic polymer (A) and polymer (B) containing, as a main component, structural units derived from an isocyanate-based crosslinking agent, as described below, and are suitable for maintaining the transparency of the pressure-sensitive adhesive sheet. Furthermore, aromatic ring-containing monomers are also suitable for adjusting the reworkability and adhesiveness of the pressure-sensitive adhesive sheet.

In the (meth)acrylic polymer (A), the content of structural units derived from copolymerization monomers is not particularly limited, and may be, for example, 0 to 40% by weight, 0.1 to 30% by weight, or 0.1 to 20% by weight.

In the (meth)acrylic polymer (A), the content of structural units derived from aromatic ring-containing monomers is not particularly limited, but is, for example, 3 to 25% by weight, more preferably 22% by weight or less, and even more preferably 20% by weight or less. This content is more preferably 8% by weight or more, and even more preferably 12% by weight or more.

In the (meth)acrylic polymer (A), the content of structural units derived from amide group-containing monomers is not particularly limited, but is, for example, 0.1 to 10% by weight, more preferably 0.2 to 8% by weight, and even more preferably 0.6 to 6% by weight.

In the (meth)acrylic polymer (A), the content of structural units derived from carboxyl group-containing monomers is not particularly limited, but is, for example, 0.1 to 25% by weight, more preferably 3% by weight or more, and even more preferably 8% by weight or more. This content is more preferably 20% by weight or less.

In addition, it is preferable that the (meth)acrylic polymer (A) have a low content of structural units derived from copolymerizable monomers having active hydrogens that are highly reactive with isocyanate-based crosslinking agents, such as hydroxyl group-containing monomers. In the (meth)acrylic polymer (A), the content of structural units derived from hydroxyl group-containing monomers is, for example, 1% by weight or less, more preferably 0.5% by weight or less, and even more preferably 0.1% by weight or less. The (meth)acrylic polymer (A) may be substantially free of structural units derived from hydroxyl group-containing monomers.

In addition to alkyl (meth)acrylates and the above-mentioned copolymerizable monomers, other copolymerizable monomers having a polymerizable functional group containing an unsaturated double bond, such as a (meth)acryloyl group or a vinyl group, can be used as the monomer component to improve the adhesiveness and heat resistance of the PSA sheet. These other copolymerizable monomers can be used alone or in combination.

Other copolymerizable monomers include, for example, acid anhydride group-containing monomers such as maleic anhydride and itaconic anhydride; caprolactone adducts of acrylic acid; sulfonic acid group-containing monomers such as allyl sulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, (meth)acrylamidopropanesulfonic acid, and sulfopropyl (meth)acrylate; phosphoric acid group-containing monomers such as 2-hydroxyethyl acryloyl phosphate; alkylaminoalkyl (meth)acrylates such as aminoethyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, and t-butylaminoethyl (meth)acrylate; and methoxyethyl (meth)acrylate. alkoxyalkyl (meth)acrylates such as N-(meth)acryloyloxymethylene succinimide, N-(meth)acryloyl-6-oxyhexamethylene succinimide, and N-(meth)acryloyl-8-oxyoctamethylene succinimide; succinimide monomers such as N-cyclohexylmaleimide, N-isopropylmaleimide, N-laurylmaleimide, and N-phenylmaleimide; maleimide monomers such as N-methylitaconimide, N-ethylitaconimide, N-butylitaconimide, N-octylitaconimide, N-2-ethylhexylitaconimide, and N-cyclohexyl Itaconimide-based monomers such as silitaconimide and N-laurylitaconimide; vinyl-based monomers such as vinyl acetate and vinyl propionate; cyanoacrylate-based monomers such as acrylonitrile and methacrylonitrile; epoxy group-containing (meth)acrylates such as glycidyl (meth)acrylate; glycol-based (meth)acrylates such as polyethylene glycol (meth)acrylate, polypropylene glycol (meth)acrylate, methoxyethylene glycol (meth)acrylate, and methoxypolypropylene glycol (meth)acrylate; tetrahydrofurfuryl (meth)acrylate, fluorine (meth)acrylate, silicone (meth)acrylate, (meth)acrylate monomers such as acrylate and 2-methoxyethyl acrylate; and silane monomers containing silicon atoms such as 3-acryloxypropyltriethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, 4-vinylbutyltrimethoxysilane, 4-vinylbutyltriethoxysilane, 8-vinyloctyltrimethoxysilane, 8-vinyloctyltriethoxysilane, 10-methacryloyloxydecyltrimethoxysilane, 10-acryloyloxydecyltrimethoxysilane, 10-methacryloyloxydecyltriethoxysilane, and 10-acryloyloxydecyltriethoxysilane.

Further examples of other copolymerizable monomers include polyfunctional monomers having two or more unsaturated double bonds, such as tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, bisphenol A diglycidyl ether di(meth)acrylate, neopentyl glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and caprolactone-modified dipentaerythritol hexa(meth)acrylate.

When other copolymerizable monomers are used as monomer components, the content of structural units derived from the other copolymerizable monomers in the (meth)acrylic polymer (A) is preferably 10% by weight or less, more preferably 7% by weight or less, and even more preferably 5% by weight or less.

The weight-average molecular weight of the (meth)acrylic polymer (A) is typically 300,000 to 4,000,000. From the standpoint of durability, the weight-average molecular weight of the (meth)acrylic polymer (A) is preferably 300,000 to 3,000,000, and more preferably 400,000 to 2,000,000. A weight-average molecular weight of 300,000 or more is preferred in terms of heat resistance. A weight-average molecular weight of 4,000,000 or less tends to make the PSA sheet less likely to harden and peel. The weight-average molecular weight (Mw)/number-average molecular weight (Mn), which represents the molecular weight distribution, is preferably 1.8 to 10, more preferably 1.8 to 7, and even more preferably 1.8 to 5. A molecular weight distribution (Mw/Mn) of 10 or less is preferred in terms of durability. The weight-average molecular weight and molecular weight distribution (Mw/Mn) are measured by GPC (gel permeation chromatography) and calculated in polystyrene equivalent terms.

The (meth)acrylic polymer (A) can be produced by known polymerization methods, such as solution polymerization, radiation polymerization using electron beams or UV rays, bulk polymerization, emulsion polymerization, and other radical polymerization methods. The resulting (meth)acrylic polymer (A) may be a random copolymer, block copolymer, graft copolymer, or the like.

In solution polymerization, for example, ethyl acetate or toluene is used as the polymerization solvent. Solution polymerization is typically carried out under reaction conditions of approximately 50 to 70°C for 5 to 30 hours, with a polymerization initiator added under a stream of inert gas such as nitrogen.

The polymerization initiator, chain transfer agent, emulsifier, etc. used in the radical polymerization are not particularly limited and can be selected appropriately. The weight-average molecular weight of the (meth)acrylic polymer (A) can be controlled by the amounts of polymerization initiator and chain transfer agent used, the reaction conditions, etc. Therefore, the amounts of polymerization initiator and chain transfer agent used are adjusted appropriately depending on the composition.

Examples of polymerization initiators include azo initiators such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[2-(5-methyl-2-imidazolin-2-yl)propane] dihydrochloride, 2,2'-azobis(2-methylpropionamidine) disulfate, 2,2'-azobis(N,N'-dimethyleneisobutylamidine), and 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]hydrate (VA-057, manufactured by Wako Pure Chemical Industries, Ltd.); persulfates such as potassium persulfate and ammonium persulfate; di(2-ethylhexyl)peroxydicarbonate, di(4-t-butylcyclohexyl)peroxydicarbonate, and di-sec-butylperoxydicarbonate. Examples of initiators include, but are not limited to, peroxide initiators such as dicarbonate, t-butyl peroxyneodecanoate, t-hexyl peroxypivalate, t-butyl peroxypivalate, dilauroyl peroxide, di-n-octanoyl peroxide, 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate, di(4-methylbenzoyl)peroxide, dibenzoyl peroxide, t-butyl peroxyisobutyrate, 1,1-di(t-hexylperoxy)cyclohexane, t-butyl hydroperoxide, and hydrogen peroxide; and redox initiators that combine peroxides and reducing agents, such as combinations of persulfates and sodium hydrogen sulfite, and combinations of peroxides and sodium ascorbate.

Polymerization initiators can be used alone or in combination, but the total amount used is preferably approximately 0.005 to 1 part by weight, and more preferably approximately 0.02 to 0.5 parts by weight, per 100 parts by weight of the monomer components.

Examples of chain transfer agents include lauryl mercaptan, glycidyl mercaptan, mercaptoacetic acid, 2-mercaptoethanol, thioglycolic acid, 2-ethylhexyl thioglycolate, and 2,3-dimercapto-1-propanol. Chain transfer agents may be used alone or in combination of two or more, but the total amount used is approximately 0.1 parts by weight or less per 100 parts by weight of the monomer component.

Examples of emulsifiers used in emulsion polymerization include anionic emulsifiers such as sodium lauryl sulfate, ammonium lauryl sulfate, sodium dodecylbenzenesulfonate, polyoxyethylene alkyl ether ammonium sulfate, and polyoxyethylene alkyl phenyl ether sodium sulfate; and nonionic emulsifiers such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenyl ethers, polyoxyethylene fatty acid esters, and polyoxyethylene-polyoxypropylene block polymers. Emulsifiers can be used alone or in combination.

Reactive emulsifiers include those incorporating radically polymerizable functional groups such as propenyl groups and allyl ether groups. Specific examples of these emulsifiers include Aqualon HS-10, HS-20, KH-10, BC-05, BC-10, and BC-20 (all manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and Adeka Reasoap SE10N (manufactured by ADEKA Corporation). Reactive emulsifiers are preferred because they are incorporated into the polymer chain after polymerization, improving water resistance. The amount of emulsifier used is preferably 0.3 to 5 parts by weight per 100 parts by weight of the total amount of monomer components, with 0.5 to 1 part by weight being more preferred from the perspective of polymerization stability and mechanical stability.

In radiation polymerization, the monomer components are polymerized by irradiating them with radiation such as electron beams or UV light to produce the (meth)acrylic polymer (A). When radiation polymerization is carried out using electron beams, it is not particularly necessary to include a photopolymerization initiator in the monomer components. When radiation polymerization is carried out using UV light, a photopolymerization initiator may be included in the monomer components, due to the advantage of being able to shorten the polymerization time. Photopolymerization initiators can be used alone or in combination.

The photopolymerization initiator is not particularly limited as long as it initiates photopolymerization, and commonly used photopolymerization initiators can be used. Examples of photopolymerization initiators that can be used include benzoin ethers, acetophenones, α-ketols, photoactive oximes, benzoins, benzils, benzophenones, ketals, and thioxanthones. The amount of photopolymerization initiator used is 0.05 to 1.5 parts by weight, and preferably 0.1 to 1 part by weight, per 100 parts by weight of the monomer component. Photopolymerization initiators can be used alone or in combination.

[Isocyanate-based crosslinking agent]
As the isocyanate-based crosslinking agent, a compound having at least two isocyanate groups (isocyanate compound) can be used. The number of isocyanate groups contained in the isocyanate compound is preferably 3 or more. The upper limit of the number of isocyanate groups is not particularly limited, and is, for example, 5. Examples of the isocyanate compound include aromatic isocyanate compounds, alicyclic isocyanate compounds, and aliphatic isocyanate compounds. It is preferable that the isocyanate-based crosslinking agent be capable of self-polymerization by reacting with water.

Examples of aromatic isocyanate compounds include phenylene diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 2,2'-diphenylmethane diisocyanate, 4,4'-diphenylmethane diisocyanate, 4,4'-toluidine diisocyanate, 4,4'-diphenyl ether diisocyanate, 4,4'-diphenyl diisocyanate, 1,5-naphthalene diisocyanate, and xylylene diisocyanate.

Examples of alicyclic isocyanate compounds include 1,3-cyclopentene diisocyanate, 1,3-cyclohexane diisocyanate, 1,4-cyclohexane diisocyanate, isophorone diisocyanate, hydrogenated diphenylmethane diisocyanate, hydrogenated xylylene diisocyanate, hydrogenated tolylene diisocyanate, and hydrogenated tetramethylxylylene diisocyanate.

Aliphatic isocyanate compounds include trimethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, pentamethylene diisocyanate, 1,2-propylene diisocyanate, 1,3-butylene diisocyanate, dodecamethylene diisocyanate, and 2,4,4-trimethylhexamethylene diisocyanate.

Examples of isocyanate-based crosslinking agents include polymers (dimers, trimers, pentamers, etc.) of the above isocyanate compounds, adducts obtained by addition to polyhydric alcohols such as trimethylolpropane, urea-modified products, biuret-modified products, allophanate-modified products, isocyanurate-modified products, carbodiimide-modified products, and urethane prepolymers obtained by addition to polyether polyols, polyester polyols, acrylic polyols, polybutadiene polyols, polyisoprene polyols, etc.

The isocyanate-based crosslinking agent is preferably an aromatic isocyanate compound or a derivative thereof, more preferably tolylene diisocyanate or a derivative thereof, in other words, a tolylene diisocyanate-based (TDI-based) crosslinking agent. From the standpoint of reactivity, TDI-based crosslinking agents are more suitable for the pressure-sensitive adhesive composition of this embodiment than xylylene diisocyanate or a derivative thereof, in other words, xylylene diisocyanate (XDI-based) crosslinking agents. In particular, the isocyanate-based crosslinking agent preferably contains an adduct of a polyhydric alcohol and tolylene diisocyanate as the TDI-based crosslinking agent. A specific example of this adduct is a trimethylolpropane/tolylene diisocyanate trimer adduct.

Commercially available isocyanate crosslinking agents include, for example, products manufactured by Tosoh Corporation under the trade names "Millionate MT," "Millionate MTL," "Millionate MR-200," "Millionate MR-400," "Coronate L," "Coronate HL," and "Coronate HX," and products manufactured by Mitsui Chemicals under the trade names "Takenate D-110N," "Takenate D-120N," "Takenate D-140N," "Takenate D-160N," "Takenate D-165N," "Takenate D-170HN," "Takenate D-178N," "Takenate 500," and "Takenate 600," with Coronate L being preferred.

The isocyanate-based crosslinking agent may be used alone or in a mixture of two or more of the above. The amount of the isocyanate-based crosslinking agent is 2 parts by weight or more, preferably 3 parts by weight or more, more preferably 5 parts by weight or more, even more preferably 8 parts by weight or more, particularly preferably 10 parts by weight or more, and may even be 11 parts by weight or more, per 100 parts by weight of the (meth)acrylic polymer (A). The amount of the isocyanate-based crosslinking agent is, for example, 30 parts by weight or less, preferably 20 parts by weight or less, more preferably less than 15 parts by weight, and even more preferably 13 parts by weight or less, per 100 parts by weight of the (meth)acrylic polymer (A). The amount of the isocyanate-based crosslinking agent is preferably 2 to 30 parts by weight per 100 parts by weight of the (meth)acrylic polymer (A).

In a pressure-sensitive adhesive composition, if the amount of isocyanate-based crosslinking agent per 100 parts by weight of (meth)acrylic polymer (A) is as high as 2 parts by weight or more, the isocyanate-based crosslinking agents may react with each other when preparing a pressure-sensitive adhesive sheet, forming polymer (B) whose main component is a structural unit derived from the isocyanate-based crosslinking agent. Polymer (B) is suitable for suppressing dimensional changes in the pressure-sensitive adhesive sheet by imparting sufficient cohesive strength to the pressure-sensitive adhesive sheet. In other words, polymer (B) is suitable for suppressing display unevenness and light leakage in image display devices. Furthermore, the combination of (meth)acrylic polymer (A) and polymer (B) is suitable for improving the durability of the pressure-sensitive adhesive sheet in high-temperature, high-humidity environments, etc.

[Peroxide-based crosslinking agent]
As the peroxide-based crosslinking agent, any peroxide that generates radical active species upon heating or light irradiation to promote crosslinking of the (meth)acrylic polymer (A) can be used as appropriate. In consideration of workability and stability, it is preferable to use a peroxide whose one-minute half-life temperature is 80°C to 160°C, and it is more preferable to use a peroxide whose one-minute half-life temperature is 90°C to 140°C.

The half-life of a peroxide is an index that indicates the decomposition rate of the peroxide, and refers to the time it takes for the remaining amount of peroxide to be reduced to half. The decomposition temperature required to obtain the half-life at a given time, and the half-life time at a given temperature, are listed in manufacturer catalogs, such as NOF Corporation's "Organic Peroxide Catalog, 9th Edition (May 2003)."

Examples of peroxides include di(2-ethylhexyl) peroxydicarbonate (1-minute half-life temperature: 90.6°C), di(4-t-butylcyclohexyl) peroxydicarbonate (1-minute half-life temperature: 92.1°C), di-sec-butyl peroxydicarbonate (1-minute half-life temperature: 92.4°C), t-butyl peroxyneodecanoate (1-minute half-life temperature: 103.5°C), t-hexyl peroxypivalate (1-minute half-life temperature: 109.1°C), t-butyl peroxypivalate (1-minute half-life temperature: 110.3°C), dilauroyl peroxide (1-minute half-life temperature: 116.4°C), and di-n-octanoyl peroxide (1-minute half-life temperature: 117.4°C). ), 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate (1-minute half-life temperature: 124.3°C), di(4-methylbenzoyl) peroxide (1-minute half-life temperature: 128.2°C), benzoyl peroxide (1-minute half-life temperature: 130.0°C), t-butylperoxyisobutyrate (1-minute half-life temperature: 136.1°C), 1,1-di(t-hexylperoxy)cyclohexane (1-minute half-life temperature: 149.2°C), etc., of which di(4-t-butylcyclohexyl)peroxydicarbonate, dilauroyl peroxide, and benzoyl peroxide are preferred due to their excellent crosslinking reaction efficiency, with benzoyl peroxide being particularly preferred.

The amount of peroxide-based crosslinking agent added is not particularly limited, and is, for example, 0.01 to 5 parts by weight, preferably 2 parts by weight or less, more preferably 1 part by weight or less, and even more preferably 0.5 parts by weight or less, per 100 parts by weight of (meth)acrylic polymer (A). When the amount of peroxide-based crosslinking agent added is within the above range, the processability and crosslinking stability of the PSA sheet can be easily adjusted.

[Other ingredients]
The pressure-sensitive adhesive composition may contain a crosslinking agent other than the isocyanate-based crosslinking agent and the peroxide-based crosslinking agent. Examples of the other crosslinking agent include an epoxy-based crosslinking agent, an imine-based crosslinking agent, and a polyfunctional metal chelate. The amount of the other crosslinking agent is preferably 2 parts by weight or less, more preferably 1 part by weight or less, per 100 parts by weight of the (meth)acrylic polymer (A). From the viewpoint of durability of the pressure-sensitive adhesive sheet, the pressure-sensitive adhesive composition may be substantially free of other crosslinking agents, particularly epoxy-based crosslinking agents.

The pressure-sensitive adhesive composition may further contain a (meth)acrylic oligomer.

The (meth)acrylic oligomer may have the same composition as the above-described (meth)acrylic polymer (A), except for the weight-average molecular weight (Mw). The weight-average molecular weight (Mw) of the (meth)acrylic oligomer is, for example, 1,000 or more, and may be 2,000 or more, 3,000 or more, or even 4,000 or more. The upper limit of the weight-average molecular weight (Mw) of the (meth)acrylic oligomer is, for example, 30,000 or less, and may be 15,000 or less, 10,000 or less, or even 7,000 or less.

(Meth)acrylic oligomers, for example, have one or more structural units derived from the following monomers: methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, alkyl (meth)acrylates such as methyl (meth)acrylate, nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, and dodecyl (meth)acrylate; esters of (meth)acrylic acid and alicyclic alcohols such as cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, and dicyclopentanyl (meth)acrylate; aromatic ring-containing (meth)acrylates such as phenyl (meth)acrylate and benzyl (meth)acrylate; and (meth)acrylates obtained from terpene compound-derived alcohols.

The (meth)acrylic oligomer preferably has structural units derived from a (meth)acrylic monomer with a relatively bulky structure. In this case, the adhesiveness of the PSA sheet can be further improved. Examples of such acrylic monomers include alkyl (meth)acrylates with an alkyl group having a branched structure, such as isobutyl (meth)acrylate and t-butyl (meth)acrylate; esters of (meth)acrylic acid and alicyclic alcohols, such as cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, and dicyclopentanyl (meth)acrylate; and aromatic ring-containing (meth)acrylates, such as phenyl (meth)acrylate and benzyl (meth)acrylate. The monomer preferably has a cyclic structure, and more preferably has two or more cyclic structures. Furthermore, when ultraviolet light is irradiated during polymerization of the (meth)acrylic oligomer and/or formation of the pressure-sensitive adhesive sheet, the above-mentioned monomer preferably does not have an unsaturated bond, as this makes it less likely that the progress of polymerization and/or formation will be inhibited. For example, alkyl (meth)acrylates having alkyl groups with a branched structure, and esters of (meth)acrylic acid and alicyclic alcohols can be used.

Specific examples of (meth)acrylic oligomers include copolymers of butyl acrylate, methyl acrylate, and acrylic acid, copolymers of cyclohexyl methacrylate and isobutyl methacrylate, copolymers of cyclohexyl methacrylate and isobornyl methacrylate, copolymers of cyclohexyl methacrylate and acryloylmorpholine, copolymers of cyclohexyl methacrylate and diethylacrylamide, copolymers of 1-adamantyl acrylate and methyl methacrylate, copolymers of dicyclopentanyl methacrylate and isobornyl methacrylate, copolymers of methyl methacrylate and at least one selected from dicyclopentanyl methacrylate, cyclohexyl methacrylate, isobornyl methacrylate, isobornyl acrylate, and cyclopentanyl methacrylate, homopolymers of dicyclopentanyl acrylate, homopolymers of 1-adamantyl methacrylate, and homopolymers of 1-adamantyl acrylate.

The polymerization method for the (meth)acrylic polymer (A) described above can be used to polymerize the (meth)acrylic oligomer.

When the pressure-sensitive adhesive composition contains a (meth)acrylic oligomer, the amount thereof may be, for example, 70 parts by weight or less, 50 parts by weight or less, or even 40 parts by weight or less, per 100 parts by weight of the (meth)acrylic polymer (A). The lower limit of the amount may be, for example, 1 part by weight or more, 2 parts by weight or more, or even 3 parts by weight or more, per 100 parts by weight of the (meth)acrylic polymer (A). The pressure-sensitive adhesive composition does not have to contain a (meth)acrylic oligomer.

The pressure-sensitive adhesive composition may further contain known additives. Examples of additives include silane coupling agents, solvents, colorants, powders such as pigments, dyes, surfactants, plasticizers, tackifiers, surface lubricants, leveling agents, rework improvers, softeners, antioxidants, anti-aging agents, light stabilizers, UV absorbers, polymerization inhibitors, inorganic or organic fillers, metal powders, particles, and foil-like materials. Furthermore, redox systems containing a reducing agent may be used within controllable limits. These additives can be used in amounts of, for example, 10 parts by weight or less, preferably 5 parts by weight or less, and more preferably 1 part by weight or less, per 100 parts by weight of the (meth)acrylic polymer (A).

Specific examples of silane coupling agents include epoxy group-containing silane coupling agents such as 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; amino group-containing silane coupling agents such as 3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, 3-triethoxysilyl-N-(1,3-dimethylbutylidene)propylamine, and N-phenyl-γ-aminopropyltrimethoxysilane; (meth)acrylic group-containing silane coupling agents such as 3-acryloxypropyltrimethoxysilane and 3-methacryloxypropyltriethoxysilane; and isocyanate group-containing silane coupling agents such as 3-isocyanatepropyltriethoxysilane.

When the pressure-sensitive adhesive composition contains a silane coupling agent, the amount thereof is, for example, 5 parts by weight or less, and may be 3 parts by weight or less, 1 part by weight or less, 0.5 parts by weight or less, 0.2 parts by weight or less, 0.1 parts by weight or less, or even 0.05 parts by weight or less, per 100 parts by weight of the (meth)acrylic polymer (A). The pressure-sensitive adhesive composition does not necessarily contain a silane coupling agent.

The type of adhesive composition may be, for example, an emulsion type, a solvent type (solution type), an active energy ray curable type (photocurable type), or a hot melt type. From the viewpoint of being able to form an adhesive sheet with excellent durability, the adhesive composition may be a solvent type. A solvent-type adhesive composition may be substantially free of a photocuring agent such as an ultraviolet curing agent.

[Characteristics of Pressure-Sensitive Adhesive Composition]
As described above, in the pressure-sensitive adhesive composition of this embodiment, the amount of isocyanate-based crosslinking agent is 2 parts by weight or more per 100 parts by weight of the (meth)acrylic polymer (A). When the amount of isocyanate-based crosslinking agent relative to the (meth)acrylic polymer (A) is this high, the initial gel fraction of a pressure-sensitive adhesive sheet produced from the pressure-sensitive adhesive composition tends to be low. However, according to the pressure-sensitive adhesive composition of this embodiment, the initial gel fraction of the pressure-sensitive adhesive sheet can be increased by further including a peroxide-based crosslinking agent. The increase in the initial gel fraction of the pressure-sensitive adhesive sheet is thought to be due to the crosslinking reaction of the (meth)acrylic polymer (A) using radicals from the peroxide-based crosslinking agent. According to the inventors' studies, a high initial gel fraction of a pressure-sensitive adhesive sheet tends to suppress the formation of dents in the pressure-sensitive adhesive sheet. Furthermore, according to the inventors' studies, pressure-sensitive adhesive sheets formed from the pressure-sensitive adhesive composition of this embodiment also tend to have improved durability.

As an example, the initial gel fraction of the adhesive sheet obtained by applying the adhesive composition to a substrate to form a coating film and then drying the coating film at 120°C for 300 seconds is 40% or more, and preferably 50% or more. There are no particular limitations on the upper limit of the initial gel fraction of the adhesive sheet, and it is, for example, 70%.

The initial gel fraction of a PSA sheet can be evaluated, for example, by the following method. First, a portion of the PSA sheet produced by the above method is scraped off within two hours of production to obtain a small piece. Next, the obtained small piece is wrapped in a stretched porous polytetrafluoroethylene membrane and tied with kite string. This results in a test piece. Next, the total weight (weight A) of the PSA sheet piece, the stretched porous membrane, and the kite string is measured. The total weight of the stretched porous membrane and the kite string used is defined as weight B. Next, the test piece is immersed in a container filled with ethyl acetate and left to stand at 23°C for one week. After standing, the test piece is removed from the container and dried in a dryer set to 130°C for two hours, and then the weight C of the test piece is measured. The initial gel fraction of the PSA sheet can be calculated from weight A, weight B, and weight C based on the following formula.
Gel fraction (wt%)=(C−B)/(A−B)×100

(Embodiments of Pressure-Sensitive Adhesive Sheet)
An example of a pressure-sensitive adhesive sheet according to the present embodiment is shown in FIG. 1 . The pressure-sensitive adhesive sheet 1 according to the present embodiment is formed from the pressure-sensitive adhesive composition described above. The pressure-sensitive adhesive sheet 1 includes, for example, a crosslinked product of a (meth)acrylic polymer (A) and a polymer (B) containing, as a main component, a structural unit derived from an isocyanate-based crosslinking agent. In the polymer (B), the content of the structural unit derived from the isocyanate-based crosslinking agent is, for example, 70% by weight or more, preferably 90% by weight or more. The polymer (B) is, for example, composed essentially of structural units derived from the isocyanate-based crosslinking agent. In the pressure-sensitive adhesive sheet 1, the crosslinked product of the (meth)acrylic polymer (A) and the polymer (B) may form an interpenetrating network (IPN) structure. This IPN structure is suitable for increasing the elastic modulus of the pressure-sensitive adhesive sheet 1 while improving its durability.

The thickness of the adhesive sheet 1 is not particularly limited and is, for example, approximately 1 to 100 μm, preferably 2 to 50 μm, more preferably 2 to 40 μm, and even more preferably 5 to 35 μm.

The method for producing the pressure-sensitive adhesive sheet 1 includes, for example, applying the above-described pressure-sensitive adhesive composition to a substrate to form a coating film, and drying the resulting coating film.

The substrate can be, for example, a release film. The pressure-sensitive adhesive sheet 1 formed on the release film can be transferred to, for example, an optical film. The substrate may also be an optical film. In this case, an optical laminate can be obtained by forming the pressure-sensitive adhesive sheet 1.

After transferring the adhesive sheet 1 to the optical film, the release film can be used as a separator until the adhesive sheet 1 is put to practical use, simplifying the process.

Examples of materials that can be used to make the release film include suitable thin sheets such as plastic film, porous materials such as paper, cloth, and nonwoven fabric, netting, foam sheets, metal foil, and laminates of these, but plastic film is preferred due to its excellent surface smoothness.

Plastic films are not particularly limited, and examples include polyethylene film, polypropylene film, polybutene film, polybutadiene film, polymethylpentene film, polyvinyl chloride film, vinyl chloride copolymer film, polyethylene terephthalate film, polybutylene terephthalate film, polyurethane film, and ethylene-vinyl acetate copolymer film.

The thickness of the release film is usually 5 to 200 μm, and preferably about 5 to 100 μm. The release film is treated with a release agent such as a silicone-based, fluorine-based, or long-chain alkyl-based agent. The release film may also be treated with a release and antifouling agent such as a fatty acid amide-based release agent or silica powder, or with an antistatic treatment such as a coating, kneading, or vapor deposition type.

A solution containing a pressure-sensitive adhesive composition (adhesive solution) may be applied to the substrate. The solids concentration of the adhesive solution is, for example, 5 to 50% by weight, and preferably 10 to 40% by weight. The adhesive solution can be prepared by adding the same solvent as the polymerization solvent or a different solvent to the pressure-sensitive adhesive composition, depending on the polymerization form of the (meth)acrylic polymer (A).

A variety of methods can be used to apply the adhesive composition to the substrate, including roll coating, kiss roll coating, gravure coating, reverse coating, roll brushing, spray coating, dip roll coating, bar coating, knife coating, air knife coating, curtain coating, lip coating, and extrusion coating using a die coater. The amount of adhesive composition applied can be adjusted appropriately depending on the desired thickness of the adhesive sheet 1.

The coating film is dried to harden it and form the adhesive sheet 1. The drying temperature for the coating film is not particularly limited and is, for example, 130°C or lower, preferably 125°C or lower, and more preferably 120°C or lower. The drying temperature for the coating film may be 60°C or higher, or may be 100°C or higher. A drying temperature of 60°C or higher tends to allow the reaction of the isocyanate-based crosslinking agent to proceed smoothly, improving the cohesive strength of the adhesive sheet 1 and reducing display unevenness in the image display device. A drying temperature of 130°C or lower tends to allow the reaction rate of the isocyanate-based crosslinking agent to be appropriately adjusted, maintaining good compatibility between the (meth)acrylic polymer (A) and the polymer (B), and ensuring transparency. The coating film may be preheated at a temperature of 90°C or lower and then dried by heating at a temperature above 90°C (e.g., 120°C).

When forming a pressure-sensitive adhesive sheet from a conventional pressure-sensitive adhesive composition containing a peroxide-based crosslinking agent, the drying temperature of the coating film must be set at a relatively high temperature (e.g., approximately 155°C) to induce thermal decomposition of the peroxide-based crosslinking agent. However, in the pressure-sensitive adhesive composition of this embodiment, the amino groups generated by the reaction of the isocyanate-based crosslinking agent can promote the thermal decomposition of the peroxide-based crosslinking agent. Therefore, with the pressure-sensitive adhesive composition of this embodiment, the crosslinking reaction by the peroxide-based crosslinking agent proceeds sufficiently even at a relatively low drying temperature of 130°C or less.

The drying time for the coated film can be adjusted appropriately depending on the composition of the adhesive composition, and is preferably 30 to 300 seconds, more preferably 40 to 240 seconds, and especially preferably 60 to 180 seconds.

[Characteristics of adhesive sheet]
It is preferable that the pressure-sensitive adhesive sheet 1 has high transparency. When the thickness of the pressure-sensitive adhesive sheet 1 is 15 μm, the haze of the pressure-sensitive adhesive sheet 1 is, for example, 1% or less, preferably 0.8% or less, and more preferably 0.5% or less.

The storage modulus G' of the pressure-sensitive adhesive sheet 1 at 25°C is not particularly limited and is, for example, 0.5 MPa or more, preferably 0.8 MPa or more, and more preferably 1.0 MPa or more. The upper limit of the storage modulus G' of the pressure-sensitive adhesive sheet 1 at 25°C is not particularly limited and is, for example, 2.0 MPa. A high-modulus pressure-sensitive adhesive sheet 1 with a storage modulus G' within the above range is suitable for suppressing dimensional changes in optical films.

The storage modulus G' of the pressure-sensitive adhesive sheet 1 at 25°C can be determined by the following method. First, a measurement sample made of the material that constitutes the pressure-sensitive adhesive sheet 1 is prepared. The measurement sample is disc-shaped. The measurement sample has a bottom diameter of 8 mm and a thickness of 2 mm. The measurement sample may be obtained by punching out a disc from a laminate in which a plurality of pressure-sensitive adhesive sheets 1 are stacked. Next, dynamic viscoelasticity measurement is performed on the measurement sample. For example, an "ARES-G2" manufactured by TA Instruments can be used for the dynamic viscoelasticity measurement. The storage modulus G' of the pressure-sensitive adhesive sheet 1 at 25°C can be determined from the results of the dynamic viscoelasticity measurement. The conditions for the dynamic viscoelasticity measurement are as follows.
Measurement conditions: Frequency: 1 Hz
Deformation mode: Torsion Measurement temperature: -70℃ to 150℃
Temperature increase rate: 5°C/min

(Embodiment of Optical Laminate)
An example of the optical laminate of this embodiment is shown in Fig. 2. The optical laminate 10A in Fig. 2 includes a pressure-sensitive adhesive sheet 1 and an optical film 2. The pressure-sensitive adhesive sheet 1 and the optical film 2 are laminated together. The optical laminate 10A can be used as an optical film with a pressure-sensitive adhesive sheet.

Examples of optical film 2 include a polarizing plate, a retardation film, and a laminate film including a polarizing plate and/or a retardation film. However, optical film 2 is not limited to the above examples. Optical film 2 may also include a glass film.

A polarizing plate is, for example, a laminate including a polarizer and a transparent protective film. The transparent protective film is, for example, arranged in contact with the main surface (the surface with the largest area) of the layered polarizer. The polarizer may also be arranged between two transparent protective films.

There are no particular limitations on the polarizer, and various types can be used. Examples of polarizers include those made by uniaxially stretching a hydrophilic polymer film, such as a polyvinyl alcohol film, a partially formalized polyvinyl alcohol film, or a partially saponified ethylene-vinyl acetate copolymer film, onto which a dichroic substance such as iodine or a dichroic dye has been adsorbed; and polyene-based oriented films, such as dehydrated polyvinyl alcohol and dehydrochlorinated polyvinyl chloride. Among these, polarizers made from a polyvinyl alcohol film and a dichroic substance such as iodine are preferred, and iodine-based polarizers containing iodine and/or iodine ions are more preferred. The thickness of the polarizer is not particularly limited, but is generally about 5 to 80 μm.

A polarizer obtained by dyeing a polyvinyl alcohol-based film with iodine and uniaxially stretching it can be produced, for example, by immersing the polyvinyl alcohol in an iodine aqueous solution, dyeing it, and then stretching it to 3 to 7 times its original length. If necessary, the polyvinyl alcohol can also be immersed in an aqueous solution of potassium iodide containing boric acid, zinc sulfate, zinc chloride, or the like. Furthermore, if necessary, the polyvinyl alcohol-based film can be immersed in water and washed before dyeing. Washing the polyvinyl alcohol-based film with water not only removes surface dirt and antiblocking agents from the polyvinyl alcohol-based film, but also swells the polyvinyl alcohol-based film, thereby preventing uneven dyeing. The polyvinyl alcohol-based film can be stretched after, during, or before dyeing with iodine. Stretching can be performed in an aqueous solution of boric acid, potassium iodide, or the like, or in a water bath.

A thin polarizer with a thickness of 10 μm or less can also be used as the polarizer. From the perspective of thinning, the polarizer thickness is preferably 1 to 7 μm. Such thin polarizers have little thickness variation, excellent visibility, and excellent durability due to little dimensional change, and are also preferable in that they allow for thinner polarizing plates.

Representative examples of thin polarizers include those described in JP 51-069644 A, JP 2000-338329 A, WO 2010/100917 A, Japanese Patent No. 4751481 A, and JP 2012-073563 A. These thin polarizers can be obtained by a manufacturing method that includes a step of stretching a polyvinyl alcohol-based resin (hereinafter also referred to as PVA-based resin) layer and a resin substrate for stretching in a laminated state, and a dyeing step. With this manufacturing method, because the PVA-based resin layer is supported by the resin substrate for stretching, defects such as breakage due to stretching can be suppressed even if the PVA-based resin layer is thin.

Among manufacturing methods that include stretching and dyeing the laminate, the manufacturing methods described in International Publication No. 2010/100917, Japanese Patent No. 4751481, and Japanese Patent Application Publication No. 2012-073563 that include a step of stretching in an aqueous boric acid solution are preferred because they allow for high stretching ratios and improve polarization performance. In particular, the manufacturing methods described in Japanese Patent Application Publication No. 4751481 and Japanese Patent Application Publication No. 2012-073563 that include a step of auxiliary in-air stretching before stretching in an aqueous boric acid solution are preferred.

The transparent protective film provided on one or both sides of the polarizer is made of a thermoplastic resin that exhibits excellent transparency, mechanical strength, thermal stability, moisture barrier properties, and isotropy. Specific examples of such thermoplastic resins include cellulose resins such as triacetyl cellulose, polyester resins, polyethersulfone resins, polysulfone resins, polycarbonate resins, polyamide resins, polyimide resins, polyolefin resins, (meth)acrylic resins, cyclic polyolefin resins (norbornene-based resins), polyarylate resins, polystyrene resins, polyvinyl alcohol resins, and mixtures thereof. The material of the transparent protective film may be a thermosetting resin or a UV-curable resin, such as a (meth)acrylic, urethane, acrylic urethane, epoxy, or silicone-based resin. When the polarizing plate has two transparent protective films, the materials of the two transparent protective films may be the same or different. For example, a transparent protective film made of a thermoplastic resin may be bonded to one major surface of the polarizer via an adhesive, and a transparent protective film made of a thermosetting resin or an ultraviolet-curable resin may be bonded to the other major surface of the polarizer. The transparent protective film may contain one or more optional additives. Examples of additives include ultraviolet absorbers, antioxidants, lubricants, plasticizers, release agents, coloring inhibitors, flame retardants, nucleating agents, antistatic agents, pigments, and colorants. The content of the thermoplastic resin in the transparent protective film is preferably 50 to 100 wt %, more preferably 50 to 99 wt %, even more preferably 60 to 98 wt %, and particularly preferably 70 to 97 wt %. When the content of the thermoplastic resin in the transparent protective film is 50 wt % or more, the inherent high transparency of the thermoplastic resin tends to be fully exhibited.

The thickness of the transparent protective film can be determined as appropriate, but is generally around 10 to 200 μm, taking into account factors such as strength, ease of handling, and thinness.

The polarizer and transparent protective film are usually adhered to each other via a water-based adhesive. Examples of water-based adhesives include isocyanate-based adhesives, polyvinyl alcohol-based adhesives, gelatin-based adhesives, vinyl latex, water-based polyurethane, and water-based polyester. Examples of adhesives other than those mentioned above include ultraviolet-curable adhesives and electron beam-curable adhesives. Electron beam-curable polarizing plate adhesives exhibit suitable adhesion to various transparent protective films. The adhesive may contain a metal compound filler.

In polarizing plates, a retardation film or the like can be formed on the polarizer instead of a transparent protective film. Another transparent protective film or a retardation film can also be formed on the transparent protective film.

The transparent protective film may have a hard coat layer on the surface opposite the surface adhered to the polarizer, and may also be treated for purposes such as anti-reflection, anti-sticking, diffusion, and anti-glare.

Retardation films can be made by stretching a polymer film or by aligning and fixing a liquid crystal material. Retardation films have birefringence, for example, in the in-plane and/or thickness directions.

Examples of retardation films include anti-reflection retardation films (see JP 2012-133303 A, paragraphs [0221], [0222], and [0228]), viewing angle compensation retardation films (see JP 2012-133303 A, paragraphs [0225] and [0226]), and tilted orientation retardation films for viewing angle compensation (see JP 2012-133303 A, paragraph [0227]).

As long as the retardation film has substantially the above-mentioned functions, there are no particular limitations on, for example, the retardation value, arrangement angle, three-dimensional birefringence, whether it is single-layer or multi-layer, etc., and any known retardation film can be used.

The thickness of the retardation film is preferably 20 μm or less, more preferably 10 μm or less, even more preferably 1 to 9 μm, and particularly preferably 3 to 8 μm.

A retardation film is composed of, for example, two layers of a quarter-wave plate and a half-wave plate in which liquid crystal material is oriented and fixed.

Another example of an optical laminate of this embodiment is shown in Figure 3. The optical laminate 10B in Figure 3 has a laminated structure in which a separator 3, a pressure-sensitive adhesive sheet 1, and an optical film 2 are laminated in this order. By peeling off the separator 3, the optical laminate 10B can be used as an optical film with a pressure-sensitive adhesive sheet.

The separator 3 can be made of suitable thin materials such as plastic films (e.g., polyethylene, polypropylene, polyethylene terephthalate, polyester film), porous materials (e.g., paper, cloth, nonwoven fabric), nets, foam sheets, metal foils, and laminates thereof. Plastic films are preferred due to their excellent surface smoothness.

The plastic film is not particularly limited as long as it is a film that can protect the pressure-sensitive adhesive sheet 1, and examples include polyethylene film, polypropylene film, polybutene film, polybutadiene film, polymethylpentene film, polyvinyl chloride film, vinyl chloride copolymer film, polyethylene terephthalate film, polybutylene terephthalate film, polyurethane film, ethylene-vinyl acetate copolymer film, etc.

The thickness of separator 3 is typically 5 to 200 μm, and preferably approximately 5 to 100 μm. Separator 3 may be treated, as needed, with a release and antifouling agent such as a silicone-based, fluorine-based, long-chain alkyl-based, or fatty acid amide-based release agent, or silica powder, or with an antistatic treatment such as a coating, kneading, or vapor deposition. In particular, by appropriately treating the surface of separator 3 with a release treatment such as silicone treatment, long-chain alkyl treatment, or fluorine treatment, releasability from adhesive sheet 1 can be further improved.

As mentioned above, the release film used when producing the pressure-sensitive adhesive sheet 1 may also be used as the separator 3.

Another example of an optical laminate of this embodiment is shown in Figure 4. The optical laminate 10C in Figure 4 has a laminated structure in which a separator 3, an adhesive sheet 1, a retardation film 2A, an interlayer adhesive 4, and a polarizing plate 2B are laminated in this order. After peeling off the separator 3, the optical laminate 10C can be used by being attached to, for example, an image-forming layer.

A known adhesive can be used for the interlayer adhesive 4. The adhesive sheet 1 may also be used as the interlayer adhesive 4.

Another example of an optical laminate of this embodiment is shown in Figure 5. The optical laminate 10D in Figure 5 has a laminated structure in which a separator 3, an adhesive sheet 1, a retardation film 2A, an interlayer adhesive 4, a polarizing plate 2B, and a protective film 5 are laminated in this order. After peeling off the separator 3, the optical laminate 10D can be used by being attached to, for example, an image-forming layer.

The protective film 5 functions to protect the optical film 2 (polarizing plate 2B), which is the outermost layer, during distribution and storage of the optical laminate 10D, and when the optical laminate 10D is incorporated into an image display device. The protective film 5 may also function as a window to the external space when incorporated into an image display device. The protective film 5 is typically a resin film. Resins constituting the protective film 5 include, for example, polyesters such as PET, polyolefins such as polyethylene and polypropylene, acrylics, cycloolefins, polyimides, and polyamides, with polyesters being preferred. However, the protective film 5 is not limited to the above examples. The protective film 5 may also be a glass film or a laminated film including a glass film. The protective film 5 may be subjected to surface treatments such as anti-glare, anti-reflection, and anti-static.

The protective film 5 may be bonded to the optical film 2 using any adhesive. Bonding using an adhesive sheet 1 is also possible.

The optical laminate of this embodiment can be distributed and stored, for example, as a rolled body obtained by rolling up a strip-shaped optical laminate, or as a sheet-shaped optical laminate.

The optical laminate of this embodiment is typically used in image display devices. Image display devices are, for example, EL displays such as liquid crystal displays, organic EL displays, and inorganic EL displays.

(Embodiment of Image Display Device)
An example of an image display device according to this embodiment is shown in Fig. 6. The image display device 11 in Fig. 6 has a laminated structure in which a substrate 7, an image forming layer (e.g., an organic EL layer or a liquid crystal layer) 6, an adhesive sheet 1, a retardation film 2A, an interlayer adhesive 4, a polarizing plate 2B, and a protective film 5 are laminated in this order. The image display device 11 includes the optical laminates 10B, 10C, and 10D shown in Figs. 3 to 5 (excluding the separator 3). The substrate 7 and the image forming layer 6 may have the same configurations as the substrate and the image forming layer, respectively, of known image display devices.

The image display device 11 in Figure 6 may be an organic EL display or a liquid crystal display. However, the image display device 11 is not limited to this example. The image display device 11 may also be an electroluminescence (EL) display, a plasma display (PD), a field emission display (FED), or the like. The image display device 11 can be used for home appliance applications, in-vehicle applications, public information displays (PID), and the like.

The image display device of this embodiment can have any configuration as long as it includes the optical laminate of this embodiment.

The present invention will be described in more detail below using examples. The present invention is not limited to the examples shown below.

[(Meth)acrylic polymer A1]
A four-neck flask equipped with a stirring blade, a thermometer, a nitrogen gas inlet tube, and a condenser was charged with a monomer mixture containing 95.1 parts by weight of butyl acrylate (BA), 4.8 parts by weight of acrylic acid (AA), and 0.1 parts by weight of 2-hydroxyethyl acrylate. Furthermore, 0.1 parts by weight of 2,2'-azobisisobutyronitrile (AIBN) as a polymerization initiator was charged together with ethyl acetate per 100 parts by weight of the monomer mixture. Nitrogen gas was introduced with gentle stirring to replace the atmosphere, and the liquid temperature in the flask was maintained at around 55°C, allowing the polymerization reaction to proceed for 7 hours. Ethyl acetate was then added to the resulting reaction solution, adjusting the solids concentration to 30%, to obtain a solution of (meth)acrylic polymer A1.

[(Meth)acrylic polymers A2 to A8]
Solutions of (meth)acrylic polymers A2 to A8 were prepared in the same manner as for (meth)acrylic polymer A1, except that the monomers used were changed as shown in Table 1.

The abbreviations in Table 1 are as follows:
BA: n-butyl acrylate BzA: benzyl acrylate NVP: N-vinylpyrrolidone AA: acrylic acid HEA: 2-hydroxyethyl acrylate HBA: 4-hydroxybutyl acrylate AIBN: azo polymerization initiator, 2,2'-azobisisobutyronitrile (Kishida Chemical Co., Ltd.)

[Preparation of adhesive sheet]
(Examples 1 to 10 and Comparative Examples 1 and 2)
A solvent-based pressure-sensitive adhesive composition was obtained by mixing a (meth)acrylic polymer and a crosslinking agent to obtain the composition shown in Table 2 below. Next, the pressure-sensitive adhesive composition was applied to the surface of a PET film, which was a base film (separator), so that the thickness of the pressure-sensitive adhesive sheet after drying would be 15 μm. A fountain coater was used to apply the pressure-sensitive adhesive composition. The resulting coating film was dried in an air-circulating constant-temperature oven at 120°C for 300 seconds, thereby forming pressure-sensitive adhesive sheets of Examples 1 to 10 and Comparative Examples 1 and 2.

The abbreviations in Table 2 are as follows:
C/L: Trimethylolpropane/tolylene diisocyanate (manufactured by Tosoh Corporation, product name: Coronate L)
TETRAD-C: 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane (manufactured by Mitsubishi Gas Chemical Company, Inc., trade name: TETRAD-C)
Peroxide: Benzoyl peroxide (manufactured by NOF Corporation, trade name: Nyper BMT)

[evaluation]
<Weight average molecular weight (Mw) of (meth)acrylic polymer>
The weight average molecular weight (Mw) of the resulting (meth)acrylic polymer was measured by GPC (gel permeation chromatography).
Analytical equipment: Tosoh Corporation, HLC-8120GPC
Column: Tosoh Corporation, G7000H _XL + GMH _XL + GMH _XL
・Column size: 7.8mmφ x 30cm each, total 90cm
Column temperature: 40°C
・Flow rate: 0.8ml/min
・Injection volume: 100μl
Eluent: tetrahydrofuran Detector: differential refractometer (RI)
・Standard sample: polystyrene

<Thickness>
The thickness of the adhesive sheet etc. was measured using a dial gauge (manufactured by Mitutoyo).

<Initial gel fraction>
A small piece of the pressure-sensitive adhesive sheet was scraped off within 2 hours of preparation, and the initial gel fraction was evaluated using the method described above. The weight of the obtained small piece was approximately 0.2 g. The expanded porous polytetrafluoroethylene membrane used was NTF1122 (average pore size: 0.2 μm) manufactured by Nitto Denko Corporation.

<Storage modulus G'>
The storage modulus G' of the PSA sheet at 25° C. was evaluated by the method described above. Dynamic viscoelasticity measurement was performed using an "ARES-G2" manufactured by TA Instruments.

<HAZE>
The haze of the pressure-sensitive adhesive sheet (thickness 15 μm) was measured in accordance with JIS K7136:1981 using a haze meter HR300 manufactured by Murakami Color Research Laboratory in an atmosphere at 25° C. The measurement was carried out with the pressure-sensitive adhesive sheet (thickness 15 μm) to be evaluated attached to a slide glass S012140 (thickness 1.3 mm) manufactured by Matsunami Glass Industry Co., Ltd.

<Humidity durability>
The humidity durability (corresponding to an accelerated durability test) of the pressure-sensitive adhesive sheet was evaluated using the following method. First, a circular polarizing plate with a pressure-sensitive adhesive sheet was formed, with one exposed surface of each of the pressure-sensitive adhesive sheets produced in the Examples and Comparative Examples. Next, the circular polarizing plate was fixed to the surface of a glass plate (Corning Eagle XG) via the pressure-sensitive adhesive sheet. The circular polarizing plate was fixed in an atmosphere of 23°C and 50% RH. Next, the plate was treated in an autoclave at 50°C and 5 atmospheres (absolute pressure) for 15 minutes, and then left to cool to 23°C to stabilize the bonding of the circular polarizing plate to the glass plate. After that, the plate was left in a heated and humidified atmosphere at 60°C and 95% RH for 500 hours. After leaving the plate, the atmosphere was returned to 23°C and 50% RH, and the presence of peeling of the circular polarizing plate from the glass plate and the formation of bubbles between the glass plate and the circular polarizing plate were visually confirmed. The humidity durability was evaluated as follows.
A: No changes in appearance such as foaming or peeling are observed.
B: Separate peeling or slight bubbling is observed at the edge, but is within a range that does not pose a problem in practical use.
C: Significant peeling or bubbling is observed at the edge, and there is a problem in practical use.

The method for forming the circular polarizing plate with adhesive sheet used to evaluate humidity durability is described below.

<Preparation of Polarizing Plate P1>
(Preparation of polarizer)
A long polyvinyl alcohol (PVA) resin film (manufactured by Kuraray, product name "PE3000," thickness: 30 μm) was uniaxially stretched in the longitudinal direction (total stretching ratio: 5.9 times) using a roll stretching machine. At the same time, the resin film was subjected to the following treatments in order: swelling, dyeing, crosslinking, washing, and drying, to produce a polarizer with a thickness of 12 μm. In the swelling treatment, the resin film was stretched 2.2 times while being treated with pure water at 20°C. In the dyeing treatment, the resin film was stretched 1.4 times while being treated with an aqueous solution at 30°C containing iodine and potassium iodide at a weight ratio of 1:7. The iodine concentration in the aqueous solution was adjusted so that the polarizer to be produced had a single transmittance of 45.0%. A two-stage crosslinking treatment was used. In the first stage of crosslinking treatment, the resin film was stretched 1.2 times while being treated with an aqueous solution at 40°C containing boric acid and potassium iodide dissolved therein. The aqueous solution used in the first crosslinking treatment had a boric acid content of 5.0 wt % and a potassium iodide content of 3.0 wt %. In the second crosslinking treatment, the resin film was stretched 1.6 times while being treated with an aqueous solution at 65°C in which boric acid and potassium iodide were dissolved. The aqueous solution used in the second crosslinking treatment had a boric acid content of 4.3 wt % and a potassium iodide content of 5.0 wt %. A potassium iodide aqueous solution at 20°C was used for the washing treatment. The potassium iodide content of the aqueous solution used for the washing treatment was 2.6 wt %. The drying treatment was carried out at 70°C for 5 minutes.

(Preparation of Polarizing Plate P1)
A triacetyl cellulose (TAC) film (Konica Minolta, product name "KC2UA", thickness 25 μm) was bonded to each main surface of the prepared polarizer using a polyvinyl alcohol adhesive. However, the TAC film bonded to one main surface had a hard coat (thickness 7 μm) formed on the main surface opposite the polarizer side. In this way, a polarizing plate P1 having a configuration of protective layer with hard coat/polarizer/protective layer (no hard coat) was obtained.

<Preparation of Retardation Film R1>
(Preparation of First Retardation Film)
26.2 parts by weight of isosorbide (ISB), 100.5 parts by weight of 9,9-[4-(2-hydroxyethoxy)phenyl]fluorene (BHEPF), 10.7 parts by weight of 1,4-cyclohexanedimethanol (1,4-CHDM), 105.1 parts by weight of diphenyl carbonate (DPC), and 0.591 parts by weight of cesium carbonate (0.2 wt% aqueous solution) as a catalyst were charged into a reaction vessel and dissolved under a nitrogen atmosphere (approximately 15 minutes). At this time, the heat transfer medium temperature in the reaction vessel was set to 150 ° C, and stirring was performed as necessary. Next, the pressure in the reaction vessel was reduced to 13.3 kPa, and the heat transfer medium temperature was increased to 190 ° C over 1 hour. Phenol generated as the heat transfer medium temperature increased was extracted from the reaction vessel (the same applies below). Next, the temperature in the reaction vessel was maintained at 190 ° C for 15 minutes, and then the pressure in the reaction vessel was changed to 6.67 kPa, and the heat medium temperature was increased to 230 ° C over 15 minutes. When the stirring torque of the agitator equipped in the reaction vessel began to increase, the heat medium temperature was increased to 250 ° C over 8 minutes, and the pressure in the reaction vessel was further reduced to 0.200 kPa or less. After reaching a predetermined stirring torque, the reaction was terminated, and the resulting reaction product was extruded into water and pelletized. In this way, a polycarbonate resin having a composition of BHEPF / ISB / 1,4-CHDM = 47.4 mol% / 37.1 mol% / 15.5 mol% was obtained. The glass transition temperature of the obtained polycarbonate resin was 136.6 ° C, and the reduced viscosity was 0.395 dL / g.

The prepared polycarbonate resin pellets were vacuum-dried at 80°C for 5 hours, and then a long resin film with a thickness of 120 μm was obtained using a film-forming device equipped with a single-screw extruder (Isuzu Chemical Engineering Co., Ltd., screw diameter 25 mm, cylinder set temperature 220°C), a T-die (width 200 mm, set temperature 220°C), a chill roll (set temperature 120-130°C), and a winder. The obtained resin film was then stretched in the width direction using a tenter stretching machine at a stretching temperature of 137-139°C and a stretch ratio of 2.5 times to obtain a first retardation film.

(Preparation of Second Retardation Film)
A liquid crystal coating solution was prepared by dissolving 20 parts by weight of a side-chain liquid crystal polymer (weight average molecular weight 5000) represented by the following chemical formula (I) (wherein 65 and 35 represent the mol% of each structural unit), 80 parts by weight of a polymerizable liquid crystal exhibiting a nematic liquid crystal phase (manufactured by BASF, trade name "Paliocolor LC242"), and 5 parts by weight of a photopolymerization initiator (manufactured by Ciba Specialty Chemicals, trade name "Irgacure 907") in 200 parts by weight of cyclopentanone. Next, the prepared liquid crystal coating solution was applied to the surface of a norbornene-based resin film (manufactured by Nippon Zeon, trade name "Zeonex"), which serves as a substrate film, using a bar coater, and then heated and dried at 80°C for 4 minutes to align the liquid crystal contained in the coating film. Next, the coating film was cured by irradiation with ultraviolet light, and a liquid crystal solidified layer (thickness 0.58 μm), which serves as a second retardation film, was formed on the substrate film. The in-plane retardation Re of the liquid crystal solidified layer for light with a wavelength of 550 nm was 0 nm, and the retardation Rth in the thickness direction was −71 nm (nx=1.5326, ny=1.5326, nz=1.6550), and the liquid crystal solidified layer exhibited refractive index characteristics of nz>nx=ny.

(Preparation of Retardation Film R1)
One surface of the first retardation film prepared above was bonded to the liquid crystal solidified layer of the second retardation film via an adhesive to prepare a retardation film R1.

<Preparation of a circularly polarizing plate with an adhesive sheet>
(Preparation of interlayer adhesive)
A four-neck flask equipped with a stirring blade, thermometer, nitrogen gas inlet tube, and condenser was charged with a monomer mixture containing 79.9 parts by weight of butyl acrylate, 15 parts by weight of benzyl acrylate, 5 parts by weight of acrylic acid, and 0.1 parts by weight of 4-hydroxybutyl acrylate. Next, 0.1 parts by weight of 2,2'-azoisobutyronitrile as a polymerization initiator was added to 100 parts by weight of the monomer mixture along with ethyl acetate. Nitrogen gas was introduced with gentle stirring to replace the atmosphere in the flask with nitrogen, and the liquid temperature in the flask was maintained at around 55°C, allowing the polymerization reaction to proceed for 7 hours. Next, ethyl acetate was added to the resulting reaction solution to adjust the solids concentration to 30% by weight, yielding a solution of a (meth)acrylic polymer to be used as an interlayer adhesive. The weight-average molecular weight of the resulting polymer was 2.2 million.

Next, 0.5 parts by weight of a trimethylolpropane/tolylene diisocyanate trimer adduct (manufactured by Tosoh Corporation, product name "Coronate L"), 0.1 parts by weight of benzoyl peroxide, a peroxide-based crosslinking agent, 0.2 parts by weight of an epoxy group-containing silane coupling agent (manufactured by Shin-Etsu Chemical Co., Ltd., product name "KBM-403"), and 0.5 parts by weight of a polyether compound having a reactive silyl group (manufactured by Kaneka Corporation, Silyl SAT10) were mixed with the resulting (meth)acrylic polymer solution, relative to 100 parts by weight of the solids content of the solution, to obtain adhesive composition PSA1, which was used as an interlayer adhesive for bonding polarizing plate P1 and retardation film R1.

(Preparation of polarizing plate with interlayer adhesive layer)
The pressure-sensitive adhesive composition PSA1 prepared above was applied to the release surface of a 38 μm thick polyethylene terephthalate (PET) film (Mitsubishi Chemical Polyester Film Co., Ltd., MRF38), which was a release film whose release surface had been silicone-treated, so that the thickness of the layer after drying would be 12 μm, and the coating was dried for 1 minute at 155° C. to form an interlayer pressure-sensitive adhesive layer. Next, the formed interlayer pressure-sensitive adhesive layer was transferred to the protective layer (without hard coat) side of polarizing plate P1, to obtain a polarizing plate with an interlayer pressure-sensitive adhesive layer.

(Preparation of a circularly polarizing plate with an adhesive sheet)
Each pressure-sensitive adhesive sheet prepared in the Examples and Comparative Examples was transferred from the release film to the second retardation film side of the retardation film R1 (the norbornene-based resin film used as the substrate film when preparing the second retardation film was peeled off). Next, the polarizing plate with the interlayer adhesive layer prepared above was attached to the first retardation film side of the retardation film R1 via the interlayer adhesive layer to obtain a circular polarizing plate with an adhesive sheet. The retardation film R1 and the polarizing plate with the interlayer adhesive layer were attached so that the angle between the slow axis of the first retardation film and the absorption axis of the polarizer was 45 degrees counterclockwise when viewed from the side of the first retardation film.

<Presence or absence of dents>
The pressure-sensitive adhesive sheet was evaluated for the presence or absence of dents by the following method. First, a retardation film R1 was produced by the above-described method. The produced pressure-sensitive adhesive sheet was attached to the main surface of the retardation film R1 together with a separator to produce a pressure-sensitive adhesive sheet-attached retardation film. The longitudinal length of the pressure-sensitive adhesive sheet-attached retardation film was 50 m or more. This polarizing plate with the pressure-sensitive adhesive sheet was wound in the longitudinal direction and left in the roll state at 23°C for 5 days. After that, the wound pressure-sensitive adhesive sheet-attached retardation film was straightened, and the surface of the pressure-sensitive adhesive sheet was visually observed to evaluate the presence or absence of dents according to the following criteria.
A: No defects in appearance B: The surface of the adhesive sheet is uneven, causing problems in practical use

As can be seen from Table 3, the pressure-sensitive adhesive sheets of the Examples, which were formed from pressure-sensitive adhesive compositions combining a sufficient amount of an isocyanate-based crosslinking agent with a peroxide-based crosslinking agent, had a higher initial gel fraction than the pressure-sensitive adhesive sheets of the Comparative Examples, and the occurrence of dents was sufficiently suppressed. Furthermore, the pressure-sensitive adhesive sheets of the Examples had superior durability compared to the pressure-sensitive adhesive sheet of Comparative Example 2, which was formed from a pressure-sensitive adhesive composition containing an epoxy-based crosslinking agent. It is presumed that the durability of the pressure-sensitive adhesive sheet of Comparative Example 2 was reduced due to excessive crosslinking of the (meth)acrylic polymer by the epoxy-based crosslinking agent.

The adhesive composition of the present invention can be suitably used to produce adhesive sheets for image display devices such as EL displays and liquid crystal displays.

1 Adhesive sheet 2 Optical films 10A, 10B, 10C, 10D Optical laminate 11 Image display device

Claims (14)

a (meth)acrylic polymer (A);
an isocyanate-based crosslinking agent in an amount of 2 parts by weight or more based on 100 parts by weight of the (meth)acrylic polymer (A);
Peroxide and
Including,
The (meth)acrylic polymer (A) contains a structural unit derived from a carboxyl group-containing monomer,
A pressure-sensitive adhesive composition , wherein the (meth)acrylic polymer (A) contains structural units derived from hydroxyl group-containing monomers in an amount of 1 wt % or less . The pressure-sensitive adhesive composition according to claim 1, wherein the amount of the isocyanate-based crosslinking agent is 2 to 30 parts by weight per 100 parts by weight of the (meth)acrylic polymer (A). The pressure-sensitive adhesive composition according to claim 1 or 2, wherein the amount of the peroxide is 0.01 to 5 parts by weight per 100 parts by weight of the (meth)acrylic polymer (A). The pressure-sensitive adhesive composition according to any one of claims 1 to 3, wherein the (meth)acrylic polymer (A) contains a structural unit derived from an aromatic ring-containing monomer. The pressure-sensitive adhesive composition according to claim 4, wherein the content of the structural units derived from the aromatic ring-containing monomer in the (meth)acrylic polymer (A) is 3 to 25% by weight. The pressure-sensitive adhesive composition according to any one of claims 1 to 5, wherein the (meth)acrylic polymer (A) contains a structural unit derived from an amide group-containing monomer. The pressure-sensitive adhesive composition according to any one of claims 1 to 6 , wherein the isocyanate-based crosslinking agent is a tolylene diisocyanate-based crosslinking agent. The pressure-sensitive adhesive composition according to any one of claims 1 to 7, wherein the pressure-sensitive adhesive composition is applied to a substrate so as to form a coating film, and the coating film is dried at 120°C for 300 seconds to obtain a pressure-sensitive adhesive sheet having an initial gel fraction of 40% or more. A pressure-sensitive adhesive sheet formed from the pressure-sensitive adhesive composition according to any one of claims 1 to 8 . The pressure-sensitive adhesive sheet according to claim 9 , which has a storage modulus G' at 25°C of 0.5 MPa or more. An optical laminate comprising the pressure-sensitive adhesive sheet according to claim 9 or 10 and an optical film. An image display device comprising the optical laminate according to claim 11 . applying the pressure-sensitive adhesive composition according to any one of claims 1 to 8 to a substrate to form a coating film;
drying the coating film;
A method for producing a pressure-sensitive adhesive sheet, comprising: The method according to claim 13 , wherein the coating film is dried at a temperature of 130°C or less.